[PowerAssert] Correctly align infix calls for built-in operators

Instead of searching for the operator in the string representation of
the whole expression, consider the operator's start to be the
first non-whitespace non-dot character _after_ the LHS of the infix
expression.

This fixes cases like this:
```
assert("Name in " in listOf("Hello", "World"))
             |       |
             |       [Hello, World]
             false
```

^KT-66208 Fixed

plugins/power-assert/testData/codegen/operator/ExcleqOperator.box.txt

@@ -5,27 +5,27 @@ assert(1 != 1)
 
 Assertion failed
 assert(" != " != " != ")
-         |
-         false
+              |
+              false
 
 Assertion failed
 assert(
     " != "
-      |
-      false
 
             !=
+            |
+            false
 
             " != "
 )
 
 Assertion failed
 assert(1/*!=*/!=/*!=*/1)
-          |
-          false
+         |
+         false
 
 Assertion failed
 assert((1 != 1) != false)
-          |
-          false
+          |     |
+          |     false
           false